Ulrich Glebe

Surface-initiated controlled radical polymerizations from silica nanoparticles, gold nanocrystals, and bionanoparticles

In recent years, core/shell nanohybrids containing a nanoparticle core and a distinct surrounding shell of polymer brushes have received extensive attention in nanoelectronics, nanophotonics, catalysis, nanopatterning, drug delivery, biosensing, and many others. From the large variety of existing polymerization methods on the one hand and strategies for grafting onto nanoparticle surfaces on the other hand, the combination of grafting-from with controlled radical polymerization (CRP) techniques has turned out to be the best suited for synthesizing these well-defined core/shell nanohybrids and is known as surface-initiated CRP. Most common among these are surface-initiated atom transfer radical polymerization (ATRP), surface-initiated reversible addition–fragmentation chain transfer (RAFT) polymerization, and surface-initiated nitroxide-mediated polymerization (NMP). This review highlights the state of the art of growing polymers from nanoparticles using surface-initiated CRP techniques. We focus on mechanistic aspects, synthetic procedures, and the formation of complex architectures as well as novel properties. From the vast number of examples of nanoparticle/polymer hybrids formed by surface-initiated CRP techniques, we present nanohybrid formation from the particularly important and most studied silica nanoparticles, gold nanocrystals, and proteins which can be regarded as bionanoparticles.

Grafting PNIPAAm from β-barrel shaped transmembrane nanopores

The research on protein-polymer conjugates by grafting from the surface of proteins has gained significant interest in the last decade. While there are many studies with globular proteins, membrane proteins have remained untouched to the best of our knowledge. In this study, we established the conjugate formation with a class of transmembrane proteins and grow polymer chains from the ferric hydroxamate uptake protein component A (FhuA; a β-barrel transmembrane protein of Escherichia coli). As the lysine residues of naturally occurring FhuA are distributed over the whole protein, FhuA was reengineered to have up to 11 lysines, distributed symmetrically in a rim on the membrane exposed side (outside) of the protein channel and exclusively above the hydrophobic region. Reengineering of FhuA ensures a polymer growth only on the outside of the β-barrel and prevents blockage of the channel as a result of the polymerization. A water-soluble initiator for controlled radical polymerization (CRP) was consecutively linked to the lysine residues of FhuA and N-isopropylacrylamide (NIPAAm) polymerized under copper-mediated CRP conditions. The conjugate formation was analyzed by using MALDI-ToF mass spectrometry, SDS-PAGE, circular dichroism spectroscopy, analytical ultracentrifugation, dynamic light scattering, transmission electron microscopy and size exclusion chromatography. Such conjugates combine the specific functions of the transmembrane proteins, like maintaining membrane potential gradients or translocation of substrates with the unique properties of synthetic polymers such as temperature and pH stimuli handles. FhuA-PNIPAAm conjugates will serve as functional nanosized building blocks for applications in targeted drug delivery, self-assembly systems, functional membranes and transmembrane protein gated nanoreactors.

Synthesis of Polystyrene and Poly(4‐vinylpyridine) Mixed Grafted Silica Nanoparticles via a Combination of ATRP and CuI‐Catalyzed Azide‐Alkyne Click Chemistry

Binary polystyrene and poly(4-vinylpyridine) mixed grafted silica nanoparticles (PSt/P4VP-g-SNPs) are fabricated using CuI -catalyzed azide-alkyne Huisgen cycloaddition (CuAAC) via grafting-to method. Azide-terminated PSt and P4VP are synthesized via post- and pre-atom transfer radical polymerization modification, respectively. Then, the polymers are simultaneously anchored onto alkyne-modified SNPs by CuAAC yielding mixed brushes as shown by Raman spectroscopy, dynamic light scattering, and thermogravimetric analysis. To the best of our knowledge, this is the first report of simultaneously grafting two distinct polymer chains to synthesize mixed grafted silica nanoparticles using CuAAC technique via grafting-to method.

Sortase-Mediated Ligation of Purely Artificial Building Blocks

Sortase A (SrtA) from Staphylococcus aureus has been often used for ligating a protein with other natural or synthetic compounds in recent years. Here we show that SrtA-mediated ligation (SML) is universally applicable for the linkage of two purely artificial building blocks. Silica nanoparticles (NPs), poly(ethylene glycol) and poly(N-isopropyl acrylamide) are chosen as synthetic building blocks. As a proof of concept, NP–polymer, NP–NP, and polymer–polymer structures are formed by SrtA catalysis. Therefore, the building blocks are equipped with the recognition sequence needed for SrtA reaction—the conserved peptide LPETG—and a pentaglycine motif. The successful formation of the reaction products is shown by means of transmission electron microscopy (TEM), matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-ToF MS), and dynamic light scattering (DLS). The sortase catalyzed linkage of artificial building blocks sets the stage for the development of a new approach to link synthetic structures in cases where their synthesis by established chemical methods is complicated.

Synthesis of thermo-responsive nanocomposites of superparamagnetic cobalt nanoparticles/poly(N-isopropylacrylamide)

Novel nanocomposites of superparamagnetic cobalt nanoparticles (Co NPs) and poly(N-isopropylacrylamide) (PNIPAM) were fabricated through surface-initiated atom-transfer radical polymerization (SI-ATRP). We firstly synthesized a functional ATRP initiator, containing an amine (as anchoring group) and a 2-bromopropionate group (SI-ATRP initiator). Oleic acid- and trioctylphosphine oxide-coated Co NPs were then modified with the initiator via ligand exchange. The process is facile and rapid for efficient surface functionalization and afterwards the Co NPs can be dispersed into polar solvent DMF without aggregation. Transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and dynamic light scattering measurements confirmed the success of ligand exchange. The following polymerization of NIPAM was conducted on the surface of Co NPs. Temperature-dependent dynamic light scattering study showed the responsive behavior of PNIPAM-coated Co NPs. The combination of superparamagnetic and thermo-responsive properties in these hybrid nanoparticles is promising for future applications e.g. in biomedicine.

CHAPTER 1:Synthetic Modifications of Proteins

In the pursuit to develop new biohybrid materials based on protein structures or peptides, peptide chemistry is a convenient approach. In addition to conventional peptide chemistry in which reactive amino acids located on the protein surface are targeted, over the past few years many new approaches have been developed. Not only new reactions able to target the protein chemically but also other disciplines such as polymer chemistry and biocatalysis are now being added to the ever-broadening portfolio of protein/peptide alterations. Lysozyme, bovine serum albumin (BSA), ferritin and proteases such as chymotrypsin are examples that have been modified via the presented approaches. Hybrid conjugates with polymers or nanoparticles, for example, can be achieved not only by covalent linkages but also by non-covalent interactions. New tools introduced into the ‘chemical toolbox’ to modify protein structures, including controlled radical polymerization [especially atom transfer radical polymerization (ATRP) and reversible addition fragmentation chain transfer (RAFT)] and ‘click’ reactions, are now commonly used to create bionanoparticle-based conjugates with new functionalities and hence associated with it new materials and applications.

Synthesis of Polystyrene-Coated Superparamagnetic and Ferromagnetic Cobalt Nanoparticles

Polystyrene-coated cobalt nanoparticles (NPs) were synthesized through a dual-stage thermolysis of cobalt carbonyl (Co2(CO)8). The amine end-functionalized polystyrene surfactants with varying molecular weight were prepared via atom-transfer radical polymerization technique. By changing the concentration of these polymeric surfactants, Co NPs with different size, size distribution, and magnetic properties were obtained. Transmission electron microscopy characterization showed that the size of Co NPs stabilized with lower molecular weight polystyrene surfactants (Mn = 2300 g/mol) varied from 12–22 nm, while the size of Co NPs coated with polystyrene of middle (Mn = 4500 g/mol) and higher molecular weight (Mn = 10,500 g/mol) showed little change around 20 nm. Magnetic measurements revealed that the small cobalt particles were superparamagnetic, while larger particles were ferromagnetic and self-assembled into 1-D chain structures. Thermogravimetric analysis revealed that the grafting density of polystyrene with lower molecular weight is high. To the best of our knowledge, this is the first study to obtain both superparamagnetic and ferromagnetic Co NPs by changing the molecular weight and concentration of polystyrene through the dual-stage decomposition method.

Enzyme–Polymer Conjugates as Robust Pickering Interfacial Biocatalysts for Efficient Biotransformations and One‐Pot Cascade Reactions

Despite the rapid development of Pickering interfacial catalysis (PIC) at liquid-liquid interfaces with chemocatalysts, the use of unstable biocatalysts at emulsion interfaces remains a technical challenge. Herein, we present a Pickering interfacial biocatalysis (PIB) platform based on robust and recyclable enzyme-polymer conjugates that act as both catalytic sites and stabilizers at the interface of Pickering emulsions. The conjugates were prepared by growing poly(N-isopropylacrylamide) on a fragile enzyme, benzaldehyde lyase, under physiological conditions. The mild in situ conjugation process preserved the enzyme structure, and the conjugates were used to emulsify a water-organic two-phase system into a stable Pickering emulsion, leading to a significantly larger interfacial area and a 270-fold improvement in catalytic performance as compared to the unemulsified two-phase system. The PIB system could be reused multiple times. Conjugates of other enzymes were also fabricated and applied for cascade reactions.